The present invention mainly corresponds to the field of food product packaging, more specifically to the meat product industry. There are a great variety of products wrapped in film for processing, handling and/or conservation. The use of a film is often complemented with a tubular net and finds a specific application in special products in which the appearance of the product is important.
The film and net assembly form a casing in which the main purpose of the film is to contain liquid parts, prevent excessive evaporation during the heat process, allow smoking, provide shine, colour and appearance to the product surface, facilitate elimination of the net and of the film itself if it is not edible, and must also be soft enough to allow the net to transmit its imprint on the product, giving it a characteristic appearance. The use of the film and net also allows using chopped meat and meat parts of various sizes and qualities, even meat emulsions and fine pastes, alone or mixed with the former in various proportions, as the film and net assembly confines and holds inside it the stuffed product until it coagulates and binds in the heat process, thereby allowing the manufacture of reconstituted meat products.
Films and tubular nets are currently applied in the food industry manually and mechanically. For this purpose, the films are supplied in cut sheets or in coils of greater or lesser length, ranging from 5 to 250 meters. Nets are supplied as rolls with minimum lengths of about 50 meters. A manual form of application consists of wrapping the food product with the films and covering it with the net, making it pass through the inside of a hollow metal cylinder or tube that supports a given amount of tubular net axially shirred or compressed on it, obtaining on the opposite end of the cylinder the product with the net that in this manner will cover it. This procedure is described in U.S. Pat. No. 4,621,482 (Crevasse, Gammon, Sullivan 1986). An even simpler procedure is described in U.S. Pat. No. 4,719,116 (Crevasse 1988).
The mechanical process is performed by “applicators”, which wrap the product simultaneously with the film and net. A typical applicator comprises a film-coil bearing roller, a device for conforming a cylinder with the film and a tube running inside the film tube formed through which the food product is made to pass. A tube is placed concentrically to this assembly in which a given quantity of net has been compressed. The end of the tube formed by the film and the end of the compressed net portion are taken jointly in front of the open end of the stuffing tube and jointly closed with a clip, knot or any suitable system, so that the system is left ready to be stuffed wither by mechanical pumping or by a manually or pneumatically actuated piston. When the product is pushed out of the tube it simultaneously pulls the film and tubular net, thereby being wrapped by both. This procedure and the device for performing it are respectively described in the U.S. Pat. No. 4,910,034 (Winkler 1990) and U.S. Pat. No. 4,958,477 (Winkler 1990). Devices for compressing the nets are also described in U.S. Pat. No. 5,273,481 (Sullivan 1993) and U.S. Pat. No. 4,924,552 (Sullivan 1990).
The full meat product packaging procedure in which the described applicators participate is comprised of the following operations:                Feeding the meat.        Pushing the meat through the tube.        Holding and shaping the piece being stuffed.        Cutting the casing.        Clipping or stapling the ends.        Extracting the stuffed piece.        Piercing the casing to eliminate air bubbles.        
These operations are in general performed in three different manners according to the type of applicator, stapling or clipping machine used and the extent of mechanisation required. The systems used are:                Systems with manual feeding and clipping.        Systems with automatic feeding and manual clipping.        Systems with automatic feeding and clipping.        
The first system is the least mechanised of the three and uses a piston applicator. After the product is stuffed it is manually collected by the other end of the tube, which is coated in the double casing, while it is pulled to thus provide he required amount of double casing. After the entire product has left the tube and it is fully covered in the double casing, the latter is held by its rear part and cut. After it is cut the piece is shaped by pressing the meat in the casing until it has the required tension and the two ends are clipped or stapled in a simple manual clipper normally located nearby. The operation is completed with the manual piercing of the film surface to eliminate air trapped during the stuffing and placing the stuffed pieces in the cart where they will be cooked. This system is the slowest of all and requires the highest manpower. One person is needed to feed the meat, another to hold and shape it and a third to clip and pierce it. The production rate is therefore generally less than 4 pieces per minute.
The second system uses a mechanical pumping applicator. The meat tube is coupled on its rear by a suitable connection system to a stuffing machine that pumps a programmed amount of meat product each time. This procedure has obvious advantages over the previous one. The first one is that uniform amounts of meat are impelled each time, the operator only having to press a button or step on a pedal, and that the amount of air trapped in the stuffed product is less, as the meat tube is always full of meat and it can only be accessed through the space between the meat tube and the net tube, this is, through the space in which the film runs. The subsequent operations of holding and shaping the pieces, cutting, clipping or stapling the ends and piercing the pieces are as described for the previous case, with the only added difficulty that as the tube remains full of meat the operator must manually strangle the stuffed piece at the end of the stuffing tube, separating the meat mass and pulling on the double casing until enough of it is exposed to cut and/or clip it, according to the method employed. The rate of this system is somewhat higher, around 5 parts per minute, and the operation can be performed by only 2 persons, one to hold, shape and cut the piece and the other to clip and pierce it.
The third system automates the feeding, pushing, shaping, clipping and cutting operations of the pieces, but not their piercing to eliminate the air. Although in the previous systems the applicators may be used with several types of stuffers and/or clippers, in this case the system constitutes an assembly integrated in a single machine that includes portioning, pushing, separation and clipping. The product is fed by an applicator coupled to a stuffing machine, with the difference that the impulsion piston is pneumatically actuated. At the outlet of the meat stuffing tube is placed the holding, shaping, cutting and clipping mechanism, which consists of a system with two pairs of double clamps, in between which is the stapler. The sequence of operations is as follows: after the meat piece has been placed inside the stuffing tube, the first pair of clamps placed immediately after the end of the stuffing tube opens to allow the latter to pass between them and through the clipper until it reaches the second pair of clamps, which are open, but not surpassing them so that the second pair can close without being hindered by the tube. The end of the tube is already closed by he double casing held by a clip. Then the piston that pushes out the meat from the tube covered by the double casing is triggered. When the piston recedes the second pair of clamps closes, strangling and holding the double casing immediately after the stuffed meat mass. At this time he tube recedes to its initial position, thereby releasing an amount of double casing with a length equivalent to this recession movement, the first pair of clamps closing to hold and collect the double casing at its height so that the stapling machine can place a pair of staples in the area included between the double clamps, simultaneously cutting the double casing between them. The piece formed in this manner resting on an inclined path of freely turning rollers falls under gravity, exiting the machine, where an operator pierces it and places it on the cart where it will be cooked. The advance of the tube between the clamps before the meat is pushed is necessary to prevent the product from colliding against these when it is stuffed and to allow the second pair to strangle the double casing, enclosing all the stuffed meat. This requires sizing the system in correspondence with the meat tube diameter. This system is the fastest of the three and can reach a rate of 6 to 7 pieces per minute, with an operator collecting and piercing the pieces, assisted part of the time by another that loads the net. The second operator normally attends to two machines, so that one can say that each machine requires 1.5 operators. This system saves double casing due to the automatic product shaping carried out by the clamps, as the piece tail ends are short and constant, their length being determined by the distance between the two pairs of clamps, which is constant. Although this system automates all stuffing operations, their rate cannot exceed 7 parts per minute as the sequence of operations described is very slow. This slowness is due to the size and weight of the moving parts, clamps, advancing and receding tube, piston, clipping machine, etc. and to its pneumatic actuation. Additionally, the machine is expensive and specific to this type of products.
Regardless of the system used, it is common to add to all the aforementioned operations the passage of the finished parts through a vacuum chamber as a complement to piercing, to help eliminate the air trapped during the stuffing, which causes important problems affecting the external appearance of the finished product and resulting in significant financial losses. This operation requires an additional operator.
In all of the above cases the diameter of the tubes through which the meat is passed is similar to that of the final product after it is closed, clipped or stapled. This implies that the cylinder formed by the film around the meat tube has a diameter similar to that of the final product and that the elastic net shirred on the outer tube is extended to the degree that it will be in the final product. The reason for this is that in this way the pouch or bag shaped assembly formed by the film and the net is open at its maximum extension and offers a lower resistance to be filled with the meat. This procedure is not lacking weak points that could be improved:
The systems hitherto developed have not attained a full and efficient automation for processing products stuffed in a double film and net casing. Machines have been developed, such as that previously described in the third system, both for stuffing entire muscles or large pieces of meat and for reconstituted products that also staple or clip the pieces automatically, but these machines are specific for this operation, as well as slow, large and costly, as they are designed to perform the separation and clipping operations of stuffed pieces through tubes whose diameter is similar to that of the finished piece. This means that it is necessary to perform the slow sequence of operations previously described, which requires a specific complex machine, large and costly. The system solves some of the drawbacks inherent to this type of elaboration, but the operation continues to be slow, not above seven pieces per minute, and the stuffed products include a great amount of surface air that is difficult and never completely eliminated.
In addition, this system does not allow a continuous stuffing that keeps the tube full of meat, as the clamps would have to close on too large a diameter of stuffed product, breaking and bursting the film due to the friction and sudden increase in pressure caused by the displacement of the large amount of meat to be separated.
As a result of the slow stuffing in both the manual and the mechanical operation the system productivity is low. This is made worse by the need for frequent stoppages to replace the tube containing the shirred or compressed net due to the limited amount of net that can be loaded on the tube. The piece piercing operation and their manipulation to subject them to a vacuum in order to remove the air simply adds manpower cost to the operation, which in the case of the most automated operation can represent up to a 60% addition to the rest of the operation.
Because of the low accuracy of the manual operation, when this is the case the weight control is poor. When a more or less viscous product made of meat parts is pumped through a large-diameter tube the possibility that one or several meat parts on the open end of the tube fall in the preceding piece introduces an element of weight variation even when the meat corresponding to each piece is independently pushed and controlled by the stuffing machine.
Oscillation in the size of the pieces in the case of the manual system are inherent to the process, while in the case of the mechanical system they are due to the difference in the net traction depending on whether the net tube has just been placed or it is about to run out of net. This difference is due to the different contact area in either case between the net and the tube and is made worse by the high tension to which the net is subjected as a result of the tube diameter.
Other oscillations are produced when loading the nets on the pneumatic loaders, when it is frequent for net portions loaded afterwards to be inserted under the previously loaded net. As the net loaded at the end is the first to be dispensed during the application, the aforementioned covered fragments will be released with greater difficulty, giving rise to short and tightened pieces with a risk of no overlapping and losing meat, or to soft and long pieces when the covered fragments are suddenly released.
Both the films and the elastic nets used to wrap meat products are expensive and constitute an important part of the production cost. Current production systems, whether manual or mechanical, are not optimised as regards using the necessary amount of these materials. In the manual operation case, the waste of material is due t the oscillation in the length of the pieces, often inevitably using more material than is required, as described above, as well as to the excess material wasted on the ends to hold the pieces as they are tightened to provide the necessary tension and clip or staple them. In the case of automatic machines, the latter aspect is controlled better as the length of the ends is controlled by the distance between the separating clamps, although this distance is greater than is necessary and is imposed by the size of the clamps.
The air trapped during the stuffing constitutes one of the most serious drawbacks of these systems. Air trapped in the stuffing is located between the film and the surface of the meat, and given the film characteristics, is not eliminated during cooking. This results in faults of the surface appearance of the products when they are released from the net for their final packaging. This is especially problematic in this type of products, which are expensive and of high quality. The areas in which the air is located are paler in colour than the rest when the products have been smoked, they are recessed and when collagen films are used these have adherence problems. Products affected by this problem must be declassified and sold at a lower price or reprocessed.
The trapped air is incorporated in the product during the stuffing due to the low pressure in which these types of products are stuffed. The air may reach the interior of the wrapping through the space between the film and the stuffing tube. The greater the tube diameter, the greater the perimeter through which the air can enter, and the lower the pressure exerted by t product on the outside opposing the entry of air. In the case of products stuffed with a piston, the stuffing operation pushes all the air in the tube in which the product must be pushed into the film and net assembly, thereby worsening the problem.
To minimise the impact of this problem, he stuffed products are pierced so that air is allowed to escape during the heat process, or previously perforated films are used. In the first case this represents an additional operation that reduces the productivity of the process and that does not solve the problem fully, while the second case may weaken the film, as well as increasing its price.
Another common problem is loss of overlapping. The mechanical problems of the process mean that the overlapping of the longitudinal film ends is frequently lost. When this occurs, the product is not correctly covered by the film, so that meat is lost and problems of many kinds arise, mainly of appearance and adherence to the net, which cannot be later removed without tearing, resulting in poor quality products.